Plasma display devices using PDPs (Plasma Display Panels) have the advantage that thinning and larger screens are possible. In the plasma display devices, images are displayed by utilizing light emission in cases where discharge cells composing pixels are discharged.
FIG. 46 is a diagram for explaining a method of driving discharge cells in an AC-type PDP. As shown in FIG. 46, the surfaces of electrodes 301 and 302 opposite to each other are respectively covered with dielectric layers 303 and 304 in the discharge cell in the AC-type PDP.
As shown in FIG. 46(a), when a voltage lower than a discharge start voltage is applied between the electrodes 301 and 302, no discharges are induced. As shown in FIG. 46(b), when a voltage in a pulse shape (a write pulse) higher than the discharge start voltage is applied between the electrodes 301 and 302, discharges are induced. When the discharges are induced negative charges are stored on a wall surface of the dielectric layer 303 after moving toward the electrode 301, and positive charges are stored on a wall surface of the dielectric layer 304 after moving toward the electrode 302. The charges stored on the wall surface of the dielectric layer 303 or 304 will be referred to as “wall charges”. Further, a voltage induced by the wall charges will be referred to as a “wall voltage”.
As shown in FIG. 46(c), the negative wall charges are stored on the wall surface of the dielectric layer 303, and the positive wall charges are stored on the wall surface of the dielectric layer 304. In this case, the polarity of the wall voltage is opposite to the polarity of an externally applied voltage. Accordingly, an effective voltage in a discharge space is lowered as the discharges progress, so that the discharges are automatically stopped.
As shown in FIG. 46(d), when the polarity of the externally applied voltage is reversed, the polarity of the wall voltage is the same as the polarity of the externally applied voltage. Accordingly, the effective voltage in the discharge space is raised. When the effective voltage at this time exceeds the discharge start voltage, discharges which are opposite in polarity to the discharges shown in FIG. 46(b) are induced. Consequently, the positive charges move toward the electrode 301, to neutralize the negative wall charges which have already been stored in the dielectric layer 303. The negative charges move toward the electrode 302, to neutralize the positive wall charges which have already been stored in the dielectric layer 304.
As shown in FIG. 46(e), the positive and negative charges are respectively stored on the wall surfaces of the dielectric layers 303 and 304. In this case, the polarity of the wall voltage is opposite to the polarity of the externally applied voltage. Accordingly, the effective voltage in the discharge space is lowered as the discharges progress, so that the discharges are stopped.
Furthermore, as shown in FIG. 46(f), when the polarity of the externally applied voltage is reversed, discharges which are opposite in polarity to the discharges shown in FIG. 46(d) are induced. Consequently, the negative charges move toward the electrode 301, and the positive charges move toward the electrode 302. The program is then returned to the state shown in FIG. 46(c).
After the discharges are thus started once by applying the high write pulse, the discharges can be continued by reversing the polarity of the externally applied voltage (sustain pulses) lower than the write pulse due to the function of the wall charges. To start discharges by applying a write pulse will be referred to as address discharges, and to continue discharges by applying sustain pulses which are alternately reversed will be referred to as sustain discharges.
Description is now made of a sustain driver in a conventional plasma display device for driving discharge cells by the above-mentioned driving method. FIG. 47 is a circuit diagram showing the configuration of the sustain driver in the conventional plasma display device.
As shown in FIG. 47, the sustain driver 600 comprises a recovery capacitor C11, a recovery coil L11, switches SW11, SW12, SW21, and SW22, and diodes D11 and D12.
The switch SW11 is connected between a power supply terminal V11 and a node N11, and the switch SW12 is connected between the node N11 and a ground terminal. A voltage Vsus is applied to the power supply terminal V11. The node N11 is connected to 480 sustain electrodes, for example. In FIG. 47, a panel capacitance Cp corresponding to all capacitances among the plurality of sustain electrodes and the ground terminal is illustrated.
The recovery capacity C11 is connected between a node N13 and the ground terminal. The switch SW21 and the diode D11 are connected in series between the node N13 and a node N12, and the diode D12 and the switch SW22 are connected in series between the node N12 and the node N13. The recovery coil L11 is connected between the node N12 and the node N11.
FIG. 48 is a timing chart showing the operation in a sustain time period of the sustain driver 600 shown in FIG. 47. In FIG. 48, a voltage at the node Nil shown in FIG. 47 and the operations of the switches SW21, SW11, SW22, and SW12 shown in FIG. 47 are illustrated.
First, in a time period Ta, the switch SW21 is turned on, and the switch SW12 is turned off. At this time, the switches SW11 and SW22 are turned off. Consequently, the voltage at the node N11 is gently raised due to LC (Inductance-Capacitance) resonance by the recovery coil L11 and the panel capacitance Cp. Then, in a time period Tb, the switch SW21 is turned off, and the switch SW11 is turned on. Consequently, the voltage at the node N11 is rapidly raised. In a time period Tc, the voltage at the node N11 is fixed to Vsus, so that sustain discharges are induced once by a discharge current supplied from the power supply terminal V11.
Then, in a time period Td, the switch SW11 is turned off, and the switch SW22 is turned on. Consequently, the voltage at the node N11 is gently lowered due to LC resonance by the recovery coil L11 and the panel capacitance Cp. Thereafter, in a time period Te, the switch SW22 is turned off, and the switch SW12 is turned on. Consequently, the voltage at the node N11 is rapidly lowered, and is fixed to a ground potential.
By repeatedly performing the above-mentioned operations in the sustain time period, periodical sustain pulses Psu are applied to the plurality of sustain electrodes, and the discharge cells are discharged when the sustain pulses Psu rise, thereby inducing sustain discharges.
As described in the foregoing, in the conventional plasma display device, the discharge cells are discharged only once when the sustain pulse rises using the sustain driver or the like, and the discharges are stopped until the subsequent sustain pulse is applied. In the discharges induced once, the discharge current is supplied from the power supply, so that a current required for the discharges is sufficiently supplied. However, ultraviolet rays are saturated with respect to the discharge current. Further, the intensity of visible light is also saturated with respect to the ultraviolet rays. Even if the discharge current is increased, therefore, luminance is hardly increased.
The conventional plasma display device is caused to emit light by thus supplying the discharge current from the power supply to induce discharges only once. Accordingly, luminous efficiency is reduced with respect to applied power. When the discharge cells are driven at such a low current level that luminance is not saturated, the discharges themselves are unstable. Consequently, the discharges cannot be repeatedly stably induced.
On the other hand, JP-A-11-282416 discloses that a second voltage Vk and a first voltage Vs (>Vk) are applied to all discharge cells which should be turned on in a sustain time period, to discharge the discharge cells having a low discharge voltage at the second voltage Vk, while discharging the discharge cells having a high discharge voltage at the first voltage Vs, thereby dispersing a discharge current. In this case, each of the discharge cells is discharged once during the half of the sustain time period. After the discharge cells having a low discharge voltage are discharged at the second voltage Vk, however, the discharge cells having a high discharge voltage are discharged at the first voltage Vs. On the whole, it seems that the discharge cells are discharged twice during the half of the sustain time period. In such discharges, however, each of the discharge cells is discharged only once. A discharge current corresponding to the whole of a PDP is merely dispersed. Accordingly, luminous efficiency cannot be improved with respect to all the discharge cells which should be turned on.
Furthermore, JP-A-11-282416, described above discloses that the second voltage Vk (≦Vs/10) and the first voltage Vs are applied to all the discharge cells which should be turned on in the sustain time period. In this case, the discharge cell having a low discharge voltage is discharged at the first voltage Vs and is discharged again at the second voltage Vk in the subsequent cycle, and the discharge cell having a high discharge voltage is discharged at the first voltage Vs and is weakly discharged again or is not discharged at the second voltage Vk in the subsequent cycle. Also in this case, therefore, all the discharge cells which should be turned on are not discharged twice during the half of the sustain time period. Some of the discharge cells are discharged only once. Accordingly, luminous efficiency cannot be improved with respect to all the discharge cells which should be turned on.
Furthermore, the conventional plasma display device is caused to emit light by supplying a discharge current from the power supply to induce discharges only once. Accordingly, luminous efficiency is reduced with respect to applied power, resulting in increased power consumption. Generally, power consumption in the plasma display device is higher than that in the other display device. It is desired that the power consumption is reduced.
When the discharge cells are driven at such a low current level that luminance is not saturated, the discharges themselves are unstable. Accordingly, the discharges cannot be repeatedly stably induced. In the PDP, various images are displayed. The number of discharge cells which are simultaneously turned on is changed, and a required discharge current is changed. When the discharge cells are driven at a low current level, the discharges are made more unstable.